A system I would propose would be that:
Development should be completely tied to ducats (monarch points honestly don't make much sense). The provinces start out with a base development level (the ones that they have at the start of the game). The development can be improved by investing ducats (lots of them). The cost becomes, of course, higher the more you develop your province (also related to what tech-group you have). In addition to this there should be a decay added, that ticks down development back to the base level the provinces had in the beginning. After a certain time (50 years or so after the development) the development that has been improved and stayed at the same level the whole time, should become the new base development for the province with every new development costs being based on this. Furthermore the temporary development levels that you added should be destroyed as soon as the province gets sieged and looted in wars or annexed by a foreign nation without cores there.
Why this changes? Because now little wallachia cannot exponentially increase their development levels just because they have a good ruler. They still can improve their development but once they are conquered, they loose most of it. It would also fix the ridiculous coring cost for newly annexed territorys.
Any thoughts on this?
Actually, since MP gain doesn't scale with development, Wallachia isn't increasing exponentially, it's increasing linearly.
On the other hand, money gain DOES scale with development, so you are actually enabling exponential growth through tying it to money gain.
Though that's not that important, but the notion of "base development" seems really, really weird to me, too. The dev level of 1444 is just that: The development it had at this arbitrary time.
And the system seems unnecessarily complicated.
A short overview of what tying it to certain things accomplishes:
MP cost offers a strong limit since it's increasing with a relatively fixed, linear rate, so as long as the dev level depends on it, it can at most rise linearly, too. It also means that small nations will be able to develop their whole land, while big nations can only develop a part of their land.
ducat cost keeps poor nations from getting unreasonable dev levels.
cooldown keeps rich OPMs from getting even more unreasonable dev levels.
As I see it, all three are necessary for a decent implementation.